The present invention relates to a steel for dies having the martensitic microstructure which has high strength and excellent machinability.
Conventionally, a pre-hardened steel for dies has been known, which is used for molding plastics, for example. The pre-hardened steel for dies is adjusted to provide with a predetermined hardness and subsequently machined to obtain a die or the like as a final product without any further quenching treatment in contrast to a usual steel for dies, which is subjected to a process of annealing, machining and quenching to increase strength (or hardness) thereof.
Thus, although the pre-hardened steel can be provided with a high hardness which ensures high strength and high wear resistance thereby applicable to a product of die or the like, it is further required to have excellent machinability which is contradictory to the former property.
As disclosed in JP-A-5-70887, JP-A-7-278737, etc., for example, there have been known materials having the above properties, which are improved to provide high hardness by precipitation effect of additive Ni, Al, Cu or the like and adjusted to have bainitic microstructure having good machinability.
The pre-hardened steel, having a metal structure whose primary microstructure is bainite, is effective in realizing high hardness and relatively good machinability.
Thus, the pre-hardened steel is not required to be subjected to quenching treatment after working and is convenient to use for die manufacturers.
However, it is necessary to control the cooling rate in the heat-treatment process for adjusting the steel to have bainitic microstructure during manufacturing products of the steel and multiple heat-treatment steps are needed disadvantageously for such adjustment to bainitic microstructure. Further, recently there is a tendency for dies to be required to have corrosion resistance as well as high strength and longer life.
On the other hand, steels whose structural primary microstructure is martensite have been used in various applications making maximum use of particular properties of the steels, the properties can be obtained by comparatively high rate cooling treatment of transformation from austenite to martensite while avoiding existence of a phase of primary ferrite, pearlite or bainite.
There are known such types of steel being applied to dies, one example of which is shown in JP-A2-3-501752 and has a chemical composition which comprises 0.01 to 0.1% C, not more than 2% Si, 0.3 to 3.0% Mn, 1 to 5% Cr, 0.1 to 1% Mo, 1 to 7% Ni, and at least one of 1.0 to 3.0% Al and 1.0 to 4.0% Cu.
It has a microstructure of lath-martensite before aging and a hardness of 30 to 38 HRC, and can be readily subjected to subsequent heat-treatment in order to improve hardness.
However, also in the case of JP-A2-3-501752, it is not taken into consideration to machine a martensitic steel having a higher hardness exceeding 38 HRC.
This is because the martensitic microstructure is considered to have a problem in machinability and because machining after adjustment to martensite with increased hardness was inconceivable.
In order to solve the above problems, the object of the present invention is to provide a high strength steel which is improved in machinability without detriment to an advantageous property of excellent balance between strength and ductility, thereby the steel can be used for dies, especially those for molding plastics, as a pre-hardened material.
With regard to the steel, the present inventors examined a relationship between machinability and toughness and also corrosion resistance and found out that machinability can be greatly improved without detriment to toughness by adjusting the steel to have an optimum chemical composition to control the martensitic microstructure transferred from austenite when quenching and precipitation behavior of intermetallic compounds and carbides during quenching and tempering, thereby the invention has been proposed.
According to the invention, there is provided a high strength steel for dies having excellent machinability, which consists essentially of, by weight, 0.005 to 0.1% C, not more than 1.5% Si, not more than 2.0% Mn, from 3.0 to less than 8.0% Cr, not more than 4.0% Ni, 0.1 to 2.0% Al, not more than 3.5% Cu, and balance of Fe and inevitable impurities including nitrogen and oxygen, and which has a metal structure whose primary microstructure is martensite, wherein nitrogen and oxygen as impurities are restricted to amount ranges of not more than 0.02% nitrogen and not more than 0.003% oxygen.
According to the invention steel, it is possible to improve heavy cutting machinability, precision electrospark machining property and high-grade polishing property by making the steel to fulfill the value defined by the following equation:
Value=(7.7xc3x97C(wt %))+(2.2xc3x97Si(wt %))+(271.2xc3x97S(wt %)) greater than 2.5,
wherein the value is more preferably not more than 6.
The invention high strength steel may comprise optionally, by weight, not more than 1% Mo, not more than 1% Co, not more than 0.5% of at least one of V and Nb, and not more than 0.20% S.